The invention is related to clay spheroids having utility, among others, as filter media, odor barrier media, and filler for low density plastics.
Commercially available lightweight spheroids for use as filter media and the like include low density ceramic particles such as cellulated glass particles and fired ceramic particles. U.S. Pat. No. 4,725,390 to Laird et al. concerns a process for making fired ceramic spheroids that are currently sold under the trademark Macrolite(copyright). Advantages of the Macrolite(copyright) spheroids include the ability to adjust the specific gravity of the spheroids so that they will either sink or float indefinitely, the strength of the spheroids, and the capability to produce the spheroids in a wide range of sizes.
The specific gravity of spheroids can be a critical property. Specific gravity is the ratio of the mass of a body to the mass of an equal volume of water at a specified temperature. Specific gravity gives an indication of how buoyant media will be in water. For example the specific gravity of Macrolite(copyright) ranges from 0.58 grams per cubic centimeter (which floats on water) to 2.05 grams per cubic centimeter (which sinks in water). U.S. patent application Ser. No. 08/807,137 now abandoned, incorporated herein by reference, discloses a filter that can be constructed using Macrolite(copyright) spheroids. The specific gravity of the spheroids is adjusted to control the backwashing rate of the filter to allow optimum particle removal without blowing media into lines above the filtration tank. U.S. patent application Ser. No. 09/361,716, incorporated herein by reference discloses a gradiated filter that uses spheroids of varying specific gravities so that the spheroids will settle into a desired layered pattern at installation and after backwashing. A related concept to specific gravity is bulk density. Bulk density is the concentration of matter, measured by the mass per unit volume, such as pounds per cubic foot. Bulk density is often used a proxy for specific gravity because it is easier to measure. In general, lower bulk density corresponds to a lower specific gravity.
In order to control the specific gravity of the spheroids in their manufacture, the process of U.S. Pat. No. 4,725,390 incorporates 0.1 to 50 parts by weight silicon carbide, an expanding agent, into the core of the spheroid. In a process known as prilling, the silicon carbide is mixed with mineral particulates and a binder to form unfired spheroids which are sorted by size and dried. The dried spheroids are then mixed with a surface metal oxide such as aluminum oxide. The metal oxide serves as a parting agent and may also provide electrical properties that attract particulate matter that a filter is designed to remove from a liquid, see U.S. patent application Ser. No. 09/235,202, incorporated herein by reference.
In the firing stage, the dried spheroids are heated in a kiln at a temperature ranging from about 1080 to 1200xc2x0 C. The temperature and length of firing are controlled to adjust the specific gravity of the spheroids. In general, the higher the temperature and longer the firing the more trapped oxidized silicon carbide gas becomes present in the spheres, lowering their specific gravity. While the Macrolite(copyright) has enjoyed great commercial success, it is desirable to produce spheroids at a lower cost using simpler manufacturing methods to provide an economical ceramic spheroid.
The present invention provides economical spheroids using a relatively simple manufacturing method. The present invention eliminates the need for mixing the expanding agent silicon carbide with mineral particulates to obtain raw spheroids for firing. Clay for the spheroids is obtained that expands when heated, without the need for additional expanding ingredients. The raw spheroids of clay can be shipped to the firing location already sorted by size and fired to obtain fired spheroids. Because the clay expands when heated, the specific gravity of the fired spheroids can be adjusted by varying the firing conditions.
The present invention provides a process of making ceramic spheroids comprising the step of firing clay spheroids comprising by weight percent: FeO (3.22-5.21), Fe3O4 (3.40-5.11), SiO2 (52.49-59.42), CaO (1.99-7.96), MgO (2.81-4.69), Al2O3 (12.05-17.65), TlO2 (0.58-0.71), MnO (0.07-0.19), Na2O (0.47-0.86), K2O (1.70-2.37), P2O3 (0.13-0.19), S (0.03-0.29), CO2 (0.90-8.23), Organic C (0.22-1.04), H2O+ (4.14-7.28), H2Oxe2x88x92 (2.12-5.48) in contact with a parting agent comprising a particulate selected from aluminum oxide, magnesium oxide, and their precursors at a sufficiently high temperature and for a sufficiently long time to form a shell.
In an exemplary embodiment of the present invention, the clay spheroids are obtained from a deposit located at Hallock in Kittson County Minn., U.S.A. The fired ceramic spheroids have a fired bulk density ranging from about ten to fifty pounds per cubic foot, however variations in process parameters may produce spheroids outside this bulk density range. The firing temperature is between 1080xc2x0 C. and 1200xc2x0 C.
In another embodiment of the present invention, the clay of the above described spheroids is used as an expanding agent with other mineral particulates, and can serve a similar function to the silicon carbide that is used in the manufacture of currently available spheroids.
Further advantages and a fuller understanding of the invention will be had from the accompanying drawings and the detailed description of the invention.